Fluid energy converting method and apparatus

ABSTRACT

There is disclosed a method and apparatus for converting the kinetic energy of a moving fluid stream into useful work by means of a cascade of thin airfoils positioned therein. In one embodiment, the airfoils are provided with at least two degrees of freedom and adjacent airfoils are movable out of phase. The airfoils are subjected to the aerodynamically induced oscillations caused by the aeroelastic phenomenon known as flutter and the oscillatory movement is then harnessed to do useful work. In an alternate embodiment, a cascade of airfoils is mechanically oscillated within a moving fluid stream to increase the propulsion of the fluid. Where the fluid is a liquid, the cascade includes a plurality of hydrofoils.

This is a division of application Ser. No. 884,816, filed Mar. 9, 1978,now U.S. Pat. No. 4,184,805, issued Jan. 22, 1980.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to techniques and apparatus for harnessingthe kinetic energy of a moving fluid stream and more particularly to acascade of airfoils or hydrofoils oscillating in a fluid stream toproduce useful work.

2. Description of the Prior Art

The recent search for alternate sources of energy has caused a renewedinterest in utilizing the inexhaustible kinetic energy of moving fluidssuch as the wind, streams and even the oceans. While the windmill is thesimplest example of a device capable of harnessing such energy, it haslimited utility because its size is restricted by the centrifugal forcesproduced with large rotating blades.

One alternative to the windmill is disclosed in U.S. Pat. No. 4,024,409to Peter R. Payne. This patent discloses a device including a wire whichis oscillated by the shedding of vortices therefrom which oscillationsare then converted into useful work. Like the aeolian harp, the TacomaNarrows Bridge and a street sign flapping in a strong gust of wind, thistype of oscillatory movement is induced when vortices are shed from ablunt body at a frequency in resonance with the natural vibrationfrequency of the object. This patent also discloses the use of a singleblade oscillated in response to wind conditions. However, like thewindmill, the amount of energy which can be harnessed by such a systemis quite limited. Furthermore, such oscillations are due to the Karmanvortex street phenomenon rather than the aeroelastic phenomenon of wingflutter.

U.S. Pat. No. 3,995,972 discloses a device including a stack of rigidlyinterconnected airfoils positioned in the wind. By sequentially varyingthe angle of attack of the airfoils, uniform oscillatory motion isproduced for reciprocating a rod which in turn drives an output device.The disadvantage of this system, like the windmill, is that there is noway to compensate for variations in wind velocity to assure asubstantially constant output.

It has also long been known that a great amount of energy is availablewhen an airfoil is subjected to the aeroelastic phenomenon of flutter.However, studies of this phenomenon have been directed solely topreventing its occurrence since, if left uncontrolled, instability andeventual destruction of the airfoil results.

Finally, some work has also been done on the production of negative dragin the case of a single oscillating airfoil. See Garrick, I.E.,Propulsion of a Flapping and Oscillating Airfoil, NACA Report No. 567,May 1936.

SUMMARY OF THE INVENTION

The present invention avoids the disadvantages of the prior art by meansof a novel method and apparatus for effectively harnessing large amountsof the available energy from a moving fluid stream with a cascade ofairfoils positioned therein. As used hereinafter, the term airfoil isintended to include a hydrofoil.

According to one aspect of the invention, there is provided a novelmethod for converting the kinetic energy of a fluid into useful work bypositioning a cascade of thin airfoils in a moving fluid stream todefine an aerodynamic system. The airfoils are at zero angle of attackwhen undisturbed and each airfoil has two degrees of freedom whileadjacent airfoils are movable out of phase. The system is then adjusteduntil the velocity of the fluid stream is a critical velocity for thesystem sufficient to induce flutter oscillations. The airfoils are thendisturbed and the resultant oscillations of the airfoils are utilized toproduce useful work. Variations in fluid velocity are detected and thesystem is controlled to maintain critical velocity and steady stateoscillations.

According to another aspect of the invention, there is providedapparatus for converting the kinetic energy of a fluid stream intouseful work comprising a support structure open at opposite ends topermit fluid flow therethrough, a plurality of thin airfoils and meansfor mounting the airfoils within the support structure in a cascade andat zero angle of attack when undisturbed. In addition, the airfoils areprovided with at least two degrees of freedom with adjacent airfoilsmovable out of phase. The apparatus further includes means for utilizingthe oscillatory movement of the airfoils to produce useful work.

The airfoils are preferably arranged in two subsystems, the airfoils ofeach subsystem being interconnected to oscillate in phase. Thesubsystems may be mechanically interconnected to move 180° out of phaseor may be interconnected solely with oppositely acting mechanicaloscillators which maintain and enhance the flutter oscillations and alsoprovide the initial disturbance of the airfoils within the fluid stream.

A control system may also be provided to maintain the flutteroscillations when the velocity of the fluid varies.

According to a further aspect of the invention, there is provided amethod for converting the kinetic energy of a fluid stream into usefulwork by positioning a device including a pair of parallel plates and athin airfoil equally spaced from each plate and having at least twodegrees of freedom within a moving fluid stream. The plates arepositioned parallel to the free stream and the airfoil is at zero angleof attack when undisturbed to define an aerodynamic system. The systemis then adjusted until the velocity of the fluid is sufficient to induceflutter oscillations, the airfoil is disturbed and the resultantoscillations are utilized to produce useful work.

According to a still further aspect of the invention, there is providedapparatus for converting the kinetic energy of a fluid stream intouseful work comprising a support structure open at opposite ends topermit fluid flow therethrough and including a plurality of equallyspaced flat plates extending parallel to the direction of fluid flow, aplurality of airfoils, means for mounting the airfoils within thesupport structure in a cascade with each airfoil having at least twodegrees of freedom and being equally spaced between adjacent flat platesat zero angle of attack when undisturbed, means interconnecting theairfoils to oscillate in phase, and means operatively associated withthe airfoils to utilize the oscillatory movement to produce useful work.

According to yet another aspect of the invention, there is providedeither a single airfoil in a bounded fluid or a cascade of airfoils in amoving fluid stream. The airfoils are mechanically oscillated toincrease the propulsion of the fluid. The mechanical driving means maybe of any type including the output from a cascade of airfoils subjectedto flutter oscillations.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto. Thoseskilled in the art will appreciate that this invention may be utilizedas a basis for designing other structures or methods for carrying outthe several purposes of this invention. It is therefore important thatthe claims be regarded as including such equivalent constructions andmethods as do not depart from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Several embodiments of the invention have been chosen for purposes ofillustration and description, as shown in the accompanying drawingsforming a part of the specification, wherein:

FIG. 1 is a perspective view of a plurality of energy converters eachincluding a cascade of airfoils according to the present invention;

FIG. 2 is a graph illustrating that, for a given set of parameters, thecritical velocity needed to induce flutter oscillations is less for acascade of airfoils than for a single airfoil;

FIG. 3 is a graph illustrating how energy can be recovered byintroducing a utility device into a system subject to flutteroscillations;

FIGS. 4 and 5 are section views taken along lines A--A of FIG. 1illustrating the position of the airfoils subjected to a criticalvelocity, before and after being disturbed;

FIG. 6 is a partial, schematic perspective view of an energy converteraccording to FIG. 1, illustrating the mounting of the airfoils accordingto a first embodiment;

FIGS. 7 and 8 are partial, schematic perspective views of an energyconverter according to claim 1, illustrating the incorporation of aelectrical network for producing alternating current;

FIG. 9 is a sectional view illustrating the electrical network forproducing alternating current;

FIG. 10 is a partial, schematic perspective view illustrating themounting of the airfoils according to a second embodiment;

FIG. 11 is a partial, schematic perspective view illustrating yetanother embodiment of the invention;

FIG. 12 is a section view illustrating an airfoil according to theembodiment of FIG. 11;

FIG. 13 is a partial, schematic perspective view illustrating a furtherembodiment of the invention;

FIG. 14 is a graph illustrating the increased efficiency achieved byoscillating a cascade of airfoils in a moving fluid stream in order toincrease propulsion;

FIG. 15 is a schematic, sectional view of one embodiment of a propulsiondevice according to the present invention; and

FIG. 16 is a schematic, sectional view of another embodiment of apropulsion device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a series of energy converters 10 according to the presentinvention and positioned in the wind. The energy converters 10 arepivotally mounted to supports 12 and assume directions transverse to thedirection of the wind by means of vanes 14.

Each energy converter 10 includes a support structure 16 open at bothends to permit free passage of the wind therethrough and has a pluralityof equally spaced thin airfoils 18₁ . . . 18₁₀ arranged within thesupport structure 16 at zero angle of attack when the airfoils areundisturbed. The uppermost and lowermost airfoils are spaced from theadjacent walls of the support structure by a distance equal to one halfthe distance between adjacent airfoils. As will be explained below, theairfoils are mounted so that when they are disturbed under conditionswhich induce flutter oscillations, adjacent airfoils oscillatesubstantially 180° out of phase and this oscillatory movement is thenutilized to produce useful work.

Each airfoil preferably has a rounded leading edge and a sharp trailingedge, has a large aspect ratio, has a symmetrical profile, i.e. has zerocamber to reduce the lift force, and is rectangular in plan form.

While a cascade of ten airfoils is shown, it should be understood thatthe number of airfoils arranged in a cascade may vary depending on theintended use of the device. In addition, while the airfoils have beenillustrated as being stacked vertically to form the cascade, they may bestacked in any direction as long as they are normally at zero angle ofattack when undisturbed without departing from the scope of theinvention.

The present invention utilizes the phenomenon of the self-excitedfeedback associated with the unsteady aeroelastic phenomenon commonlyknown as aircraft wing flutter. This phenomenon involves the interactionof the elastic, inertia and dissipative forces of the airfoil with theunsteady aerodynamic forces resulting from the movement of an airfoil ina fluid stream. As the airfoil oscillates in pitch (angular rotation α)and plunge (translational movement h), there results a complexgeneration of alternating vortices from the trailing edge which in turnform a trailing wake. The vorticity of the wake then feeds back to theairfoil to introduce a force and movement having components of plus orminus 90° out of phase with the airfoil motion. This 90° out of phasecomponent introduces an apparent damping to the airfoil. At a criticalvelocity (V_(c)), this aerodynamic damping component becomes negativeand balances the positive mechanical damping of the oscillating airfoil,to provide harmonic oscillations of the airfoil. However, at velocitiesabove this critical velocity, increases in aerodynamic energy producegreat instability and eventually the aerodynamic energy becomes so greatthat the airfoil is destroyed.

Much study has been done on the flutter phenomenon. While the vastamount of energy available during flutter has long been recognized,research has been directed solely to avoiding its disastrousconsequences since during aircraft flight such flutter oscillationscannot be controlled but instead continue to build until the aircraftwing is eventually destroyed. The present invention, however, is able toutilize the flutter phenomenon because there is provided a controlsystem which prevents the instability and destruction which normallyresult in wing flutter situations.

The use of a cascade provides a distinct advantage over a single airfoilsubjected to flutter. It can be shown that for a particular set ofparameters, the critical velocity needed to oscillate a cascade is lessthan that for a single airfoil. Furthermore, when the airfoils arearranged so that adjacent airfoils oscillate 180° out of phase, thecritical velocity for a particular set of parameters will be the lowestpossible.

FIG. 2 is a graph illustrating advantage of the cascade. The ordinate isa non-dimensional ratio of the critical velocity for a cascade V(s/c)where adjacent airfoils oscillate 180° out of phase to the criticalvelocity for a single airfoil V(∞). The parameter s/c defines thespacing of airfoils arranged in a cascade where s is the distancebetween adjacent airfoils and c is the chord (see FIG. 4). Thisparameter forms the abscissa in FIG. 2. For a particular set ofparameters, it can be seen that where s/c is about 1/3, the criticalvelocity for a cascade is about 1/2 that needed to produce flutter inthe case of a single airfoil. Accordingly by using a cascade for theabsorption of energy due to the flutter phenomenon, flutter can beachieved at much lower velocities than with a single airfoil. In fact,by controlling the various parameters as will be explained hereinafter,the critical velocity needed to induce harmonic oscillations can be aslow as 1 m.p.h.

FIG. 3 is a graph illustrating the advantages of incorporating a utilitydevice such as an electrical network into an oscillating cascade. Theordinate is a non-dimensional flutter speed V/ω.sub.α ·b where V is thevelocity, ω.sub.α is the natural vibration frequency associated withpure pitch when V=0 and b is the airfoil semi-chord. The abscissa is theratio of ω_(h) to ω.sub.α, where ω_(h) is the natural frequencyassociated with pure plunge when V=0. The region below each curvedefines the region of stability where the aerodynamic energy E_(A) isless than the mechanical energy E_(M). For a given set of values ω_(h)and ω.sub.α, the critical velocity V_(c) can be derived. Above eachcurve, which represents the critical velocity V_(c), E_(A) is greaterthan E_(M) and, accordingly, defines a region of instability. The lowercurve represents the critical velocity for a single airfoil while theupper curve represents the critical velocity where a utility device isincorporated into the system. This graph then illustrates that by addinga utility device, the aerodynamic energy which would otherwise cause thesystem to enter the unstable region is instead absorbed by the utilitydevice while the system remains in the stable region. In the case of acascade, since critical velocity is lower for a particular set ofparameters, the associated curves will, of course, be lower than thoseshown in FIG. 3 for the single airfoil.

FIGS. 4 and 5 illustrate the harmonic oscillations which result when acascade of airfoils is subjected to fluid flow at critical velocity,i.e. at flutter. In FIG. 4, the airfoils are at zero angle of attackprior to being disturbed. Even though the wind velocity is at thecritical level, there is no movement of the airfoils due to theirsymmetrical contour. However, as soon as the system is disturbed bymoving at least one of the airfoils, the flow field has the effect ofcoupling adjacent airfoils so that they begin to oscillate approximately180° out of phase as shown schematically in FIG. 5. This oscillatorymovement, which occurs at critical velocity, was first observed duringstudies of the adverse effects of flutter on rotating compressor blades.

Turning to FIG. 6, there is shown a first embodiment for mounting theairfoils of the present invention within the support structure 16 topermit the substantially 180° out of phase oscillations produced atflutter.

In order to facilitate understanding of the invention, only four of theten airfoils shown in FIG. 1 are depicted in FIG. 6, namely, airfoils18₄ -18₇ although all of the airfoils will be referred to hereinafter.As shown, the airfoils are arranged to define two subsystems, the oddnumbered airfoils 18₁, 18₃, 18₅, 18₇ and 18₉ defining a first subsystemand the even numbered airfoils 18₂, 18₄, 18₆, 18₈ and 18₁₀ defining asecond subsystem. The airfoils of each subsystem are interconnected tomove in phase while each airfoil has at least two degrees of freedomsince at least two degrees of freedom are required to induce flutteroscillations.

For the first subsystem, a pair of horizontal bars 20a, 20b are mountedto the bottom wall 22 of the support structure by springs 23a, 23b,while a second pair of horizontal bars 24a, 24b are mounted by springs25a, 25b to the upper wall 26 of support structure 16. A pair of rods28a, 28b extend between bars 20a and 24a and 20b and 24b, respectively,and are pivotally connected at 29a, 29b to the leading edge of each oddnumbered airfoil. A similar pair of rods 30a, 30b extend between bars20a, 24a and 20b, 24b respectively, and are pivotally connected at 31a,31b to the trailing edge of each odd numbered airfoil. Rods 28a, 28b,30a and 30b are connected to be slidable along their associatedhorizontal bars in order to accommodate limited pitching movement of theinterconnected odd numbered airfoils without binding. Stops 32a, 32b arealso provided to limit the amount of movement of this subsystem.Accordingly, with this mounting arrangement, a limited amount of pitchand plunge is permitted for the airfoils of the first subsystem.

The second subsystem is connected to the support structure 16 in asimilar fashion. Thus, horizontal bars 34a, 34b and 36a, 36b aresupported respectively by springs 37a, 37b, 38a, 38b and areinterconnected by rods 40a, 40b and 42a, 42b, which are pivotallyconnected at 43a, 43b and 44a, 44b to the leading and trailing edgesrespectively of the even numbered airfoils. Rods 43a, 43b, 44a, 44b areconnected to their associated horizontal bars to permit limited pitchingmovement. Finally, stops 45a, 45b are also provided to limit themovement of the even numbered airfoils.

From the foregoing, it will be apparent that the two subsystems are freeto oscillate in pitch and plunge relative to each other and that whenthe energy converting device 10 is subjected to the wind at a criticalvelocity, the adjacent airfoils will oscillate approximately 180° out ofphase as shown in FIG. 5. Thus the critical velocity will be the lowestpossible for a particular set of parameters.

Since it is necessary to disturb the airfoils in order to achieve thedesired harmonic oscillations, at least one mechanical oscillator isprovided for one of the subsystems. In the embodiment of FIG. 6,oscillators 46 are provided for each subsystem with an oscillator at theforward and rearward end of each horizontal bar 20a, 20b, 34a, 34b. Theforward and rearward oscillators for each subsystem are operated 180°out of phase. Similarly the corresponding oscillators for each subsystemare also oscillated 180° out of phase. In addition to providing theinitial disturbance, operation of these oscillators maintains andenhances the oscillatory movement. Thus if the wind should die down to avelocity which is too low for the control system, which will beexplained below, to maintain the system at critical velocity, themechanical oscillators will keep the airfoils oscillating, until thevelocity increases sufficiently to reestablish flow at criticalvelocity.

With the present invention, the natural velocity of the wind is used asthe critical velocity. Accordingly, in order for this velocity to induceflutter oscillations, one or more parameters of the system, whichincludes the energy converter and the wind, must be varied. Furthermore,because the velocity of the wind may vary with time, these parametersmust be varied in response to changes in velocity so that the prevailingvelocity will maintain the oscillations.

To vary the parameters of the energy converter 10 in accordance with thewind velocity, there is provided a control system which includes adetector (not shown) such as an anemometer for detecting wind velocityor one which detects the amplitude of the oscillations. A signal fromthis detector is fed back to the energy converter 10 to vary at leastone parameter thereof. Since critical velocity depends on the rigidityof the airfoils and the location of the center of gravity, theseparameters can be varied by varying the spring stiffness or by moving amovable mass. In the present embodiment, a plurality of movable masses Mare mounted along bars 20a, 20b, 34a, 34b. These masses are movablealong their respective bars to adjust the system until the prevailingwind velocity is the critical velocity for the system. Thereafter, inresponse to the detection signals, these masses are movable to adjustthe system so that the velocity remains critical and the oscillationscontinue. Alternatively, the stiffness of the springs supporting thehorizontal bars, the frequency or amplitude of the oscillations by themechanical oscillators or a parameter associated with a utility device,for example, the resistance in an electrical network, can be varied.

While in the foregoing, the parameters associated with the energyconverter are controlled, it is also within the scope of the inventionto control fluctuations in wind velocity so that a constant velocity iscontinuously applied to the airfoils.

FIGS. 7-9 show a utility device for converting the oscillatory movementof the airfoils into useful work. In these figures an electrical networkis incorporated into the system to produce alternating current from theoscillatory movement. Each airfoil is provided with an electrical coil41 and a magnet 47 attached to the sidewall 23 of the support structure16. As the airfoils oscillate, alternating electrical current is inducedin the coils 41. However, it is also within the scope of the presentinvention to position the magnets 47 on the airfoils while attaching thecoils 41 on the sidewall 23.

Alternatively, since the airfoils of each subsystem are mechanicallyinterconnected, the energy absorbed by each airfoil of a subsystem canbe transferred to a single utility device. For example, a singleelectrical network can be connected to each subsystem. Thus a magnet 47or coil 41 can be connected to one of the horizontal bars associatedwith each subsystem with the other element connected to the adjacentwall of the support structure 16. The advantage of this construction isthat the electrical networks are spaced from the oscillating airfoilsand cannot substantially affect the flow pattern past the airfoils.

The oscillatory movement of the airfoils caused by the wind can also beutilized to do other types of useful work such as, for example,operating a pump.

FIG. 10 illustrates a second embodiment of an energy converter 10according to the present invention with similar reference numeralsdesignating similar elements. Unlike the embodiment of FIG. 6 where thetwo subsystems are interconnected only through the oppositely appliedoutputs of oscillators 46, in the present embodiment the subsystems aremechanically interconnected to assure that they always operate 180° outof phase. Connecting bars 48a, 49a, pivotally interconnect horizontalbars 20a and 34a, and are also pivotally connected to the bottom wall 22by pivot pins 50a, 51a while connecting bars 48b, 49b pivotallyinterconnect horizontal bars 20b, 34b and are pivotally connected tobottom wall 22 through pivot pins 50b, 51b. At the upper end of theenergy converter, connecting bars 52a, 53a pivotally interconnecthorizontal bars 24a and 36a and are pivotally connected to the upperwall 26 at 54a and 55a, respectively, while connecting bars 52b, 53bpivotally interconnect horizontal bars 24 b and 36b and are pivotallyconnected to the upper wall 26 at 54b and 55b, respectively. The pivotalconnections must be such as to permit the limited pitching and plungingmovement at flutter. From the foregoing, it will be apparent that in theembodiment of FIG. 10, the two subsystems are mechanicallyinterconnected to move 180° out of phase. With such an arrangement theairfoils cannot diverge from the ideal, out-of-phase oscillatory motiondepicted in FIG. 5.

A further embodiment is disclosed in FIGS. 11 and 12 wherein similarreference numerals are used to identify similar elements. While in theprevious embodiments, the airfoils are freely movable in pitch andplunge, in the instant embodiment the airfoils are only movable in pitchwhile separate flaps 56₁ . . . 56₁₀ are pivotally connected to therespective airfoils 18₁ . . . 18₁₀ to provide the second degree offreedom.

Each airfoil is provided with pins 58a and 58b at opposite ends formounting the airfoils to the sidewalls of the support structure 16.These pins permit pitching movement and also support the airfoils toprevent plunging movement. Since the rods do not support the airfoils inthis embodiment, a single set of rods, adjacent one side of the energyconverter is sufficient to interconnect each subsystem. Thus horizontalbars 60, 62 are supported by springs 64, 66 and are interconnected byrods 68 and 70 which are pivotally connected at 72 and 74 to the leadingand trailing edges of the even numbered airfoils. Stops 76 and 78 areprovided to limit the amount of pitching movement.

Similarly, on the other side of the energy converter 10, horizontal bars80 and 82 are supported by springs 64 and 66 and are interconnected byrods 84 and 86 which are pivotally connected at 88 and 90 to the leadingand trailing edges of the odd numbered airfoils. Stops 76 and 78 arealso provided for this subsystem. Of course, the subsystems may also beinterconnected mechanically as in the embodiment of FIG. 10.

Each flap 56₁ . . . 56₁₀ is pivotally connected to its associatedairfoil. While the flaps are spring biased to assume the position shownin FIG. 12, they may be moved in directions B-C due to the aerodynamiceffects of the fluid stream as the airfoil oscillates in pitch indirections D-E.

FIG. 13 illustrates yet another embodiment of the present invention withsimilar reference numerals used for similar elements. In the foregoingembodiments, the energy converter 10 has included two subsystems ofairfoils which move substantially 180° out of phase. It can be shownthat a single airfoil which is equally spaced between a pair of flatplates when in an undisturbed state acts as an infinite cascade. Thiswill be apparent from the following.

In the case of two subsystems oscillating 180° out of phase, withadjacent airfoils spaced apart by a distance s, the fluid is undisturbedat s/2. The same type of flow results if, instead of a cascade ofairfoils, a flat plate is positioned at s/2 both above and below theairfoil.

In FIG. 13, there is provided a cascade of airfoils 18₁ . . . 18₁₀equally spaced apart by the distance s. While not shown, it will beapparent that airfoil 18₁ is spaced from the bottom wall 22 by adistance s/2 while airfoil 18₁₀ is spaced from upper wall 26 by distances/2. Flat plates 92 are positioned halfway between the airfoils in theundisturbed state, i.e. s/2 and the airfoils are pivotallyinterconnected at 94, 96 to rods 98a, 98b, 99a, 99b, which in turn areconnected for limited sliding movement to horizontal bars 100a, 100b,102a, 102b. As in the previous embodiments, the horizontal bars arespaced from the upper and bottom walls of support structure 16 bysprings 103.

It will be apparent that each bounded airfoil acts as an infinitecascade, i.e. has the lowest possible critical velocity for a particularset of parameters. In addition, since the airfoils are interconnected tomove in phase in both pitch and plunge through the rods 98a, 98b, 99a,99b, the energy absorbed by each airfoil may be applied to a singleutility device.

While not illustrated, the embodiments of FIGS. 10, 12 and 13 are alsoprovided with means for adjusting the parameters of the system such asmovable masses M, a control system responsive to changes in fluidvelocity and mechanical oscillators 46 to maintain and enhance theflutter oscillations.

Similarly it will be appreciated that all of the foregoing embodimentsmay be constructed to permit three or more degrees of freedom since theinvention is not intended to be limited to only two degrees of freedom.For example, flaps may be provided in combination with airfoils whichare freely movable in both pitch and plunge to provide an energyconverter 10 having three degrees of freedom.

In operation, the energy converter 10 is positioned at a location whereit will be subject to the action of the wind. Thereafter, based on thevelocity of the wind, various parameters are adjusted so that the windvelocity will be a critical velocity for the system. Then, at least oneairfoil is disturbed to initiate harmonic oscillations and the utilitydevice harnesses the energy from the wind and converts it into usefulwork such as the production of alternating current or pumping action.Because a control system is provided, variations in the velocity of thewind will be detected and the energy converter will be automaticallyadjusted so that the prevailing velocity continues to cause harmonicoscillations due to flutter.

While the energy converter 10 is shown as being positioned in the wind,the present invention is also directed to a method and apparatus whereinthe moving fluid is a liquid such as water.

Thus far the invention has been disclosed as including a cascade ofairfoils driven solely by a moving fluid stream to do useful work.However, the cascade can also be mechanically oscillated so that theenergy of the moving fluid stream is utilized to increase its ownpropulsion.

In 1936, I. E. Garrick reported that by oscillating a single airfoil ina moving fluid, a negative drag is produced. FIG. 14 is a graphillustrating this phenomenon. The ordinate is the ratio of the averagework done in unit time (P_(X) V) by the propulsive force to the averagework done in unit time W to maintain the oscillations against theaerodynamic forces and pitching moment. The abscissa is anon-dimensional relationship V/ωb where V is the velocity, ω is thefrequency of oscillation in pure plunge and b is the semi-chord. Curve104 represents Garrick's findings for a single airfoil in pure plungewhere s/c=∞. The case of a cascade where s/c=1/3 is represented by curve105.

By way of example, assume that V=10 ft./sec. b=1/2 ft, ω=40 rad/sec. sothat V/ωb =0.5. From FIG. 14 it will be seen that for the single airfoil(s/c=∞) P_(X) V/W =0.53 while for the cascade (s/c=1/3) P_(x) V/W=0.9.Thus by oscillating a cascade in a moving fluid stream, the efficiencyof the propulsive energy derived will be 1.7 times that for a singleairfoil.

Similar increases in efficiency can be achieved where the cascade isoscillated in pure pitch, a combination of pitch and plunge or whereflaps are provided either alone or in combination with pitch and/orplunge.

Since the cascade is here being used only to assist in the propulsion ofthe fluid in which the cascade is positioned, flutter and criticalvelocity are not important factors.

FIG. 15 shows one embodiment for increasing the propulsion of a movingfluid stream. A cascade of airfoils 106 is arranged to define twosubsystems within a fluid stream 108 which is being pumped through aconduit 109. The sybsystems are oscillated 180° out of phase by twomechanical driving sources 110, 112 in pure plunge.

Driving sources 110, 112 can be of any type. Thus it is within the scopeof the present invention to use a fluid-driven cascade at flutter toproduce mechanical movement for oscillating the airfoils. In such aconstruction a first cascade is provided in a first fluid streammaintained at critical velocity and the harmonic oscillations of thecascade are then used to oscillate a second cascade to thereby increasethe propulsion of the second fluid stream.

FIG. 16 illustrates another embodiment of a device for increasing thepropulsion of a moving fluid stream. As in the embodiment of FIG. 15,the fluid stream 108 is pumped through a conduit 109 and the airfoilsare arranged in two subsystems, each responsive to one of two mechanicaldriving sources 110, 112 operating 180° out of phase. In thisembodiment, each subsystem is oscillated in both pitch and plunge.

Since as stated above, a single airfoil equidistant from the oppositewalls of a bounded fluid acts as an infinite cascade, a single airfoilcan be oscillated in a bounded fluid to increase propulsion.

While the cascade of airfoils in FIGS. 15 and 16 are shown positionedwithin a bounded fluid, it is also within the scope of the invention toutilize such a cascade in a naturally flowing unbounded fluid stream,for example, a river or the wind.

Having thus described the invention with particular reference to thepreferred forms thereof, it will be obvious to those skilled in the artto which the invention pertains, after understanding the invention, thatvarious changes and modifications may be made therein without departingfrom the scope of the invention as defined by the claims appendedhereto.

What is claimed is:
 1. A method for converting the kinetic energy of afluid stream into useful work comprising the steps of positioning adevice including a cascade of thin airfoils in a moving fluid stream todefine an aerodynamic system wherein the airfoils are at zero angle ofattack when undisturbed and are provided with at least two degrees offreedom and wherein adjacent airfoils are movable out of phase,adjusting the system until the velocity of the fluid stream is acritical velocity for the system sufficient to induce flutteroscillations, disturbing at least one of the airfoils, and thenutilizing the resultant oscillations to produce useful work.
 2. A methodaccording to claim 1, wherein the device includes means for adjustingthe parameters associated with the device, and wherein said adjustingstep includes operating said adjusting means.
 3. A method according toclaim 2, further including the step of controlling the operation of saidadjusting means in response to variations in fluid velocity to maintainthe system at critical velocity.
 4. A method according to claim 1,further including the step of applying mechanical oscillations to theairfoils to maintain and enhance the flutter oscillations.
 5. A methodaccording to claim 1, wherein the oscillations of the airfoils are usedto produce alternating current.
 6. A method according to claim 1,wherein the oscillations of the airfoils are utilized to operate a pump.7. A method according to claim 6, wherein the pump includes a secondcascade of airfoils in another fluid stream and wherein the oscillationsof the airfoils of the first cascade are utilized to oscillate theairfoils of the second cascade.
 8. Apparatus for converting the kineticenergy of a fluid stream into useful work comprising a support structureopen at opposite ends to permit fluid flow therethrough, a plurality ofthin airfoils, means for mounting said airfoils within said supportstructure in a cascade at zero angle of attack when undisturbed and sothat each airfoil has at least two degrees of freedom while adjacentairfoils are movable out of phase, and means operatively associated withsaid airfoils for utilizing the oscillations thereof to produce usefulwork.
 9. Apparatus according to claim 8, wherein said airfoils have zerocamber.
 10. Apparatus according to claim 8, wherein said airfoils have ahigh aspect ratio and are rectangular in plan form.
 11. Apparatusaccording to claim 8, further including means to disturb at least one ofsaid airfoils.
 12. Apparatus according to claim 8, further including avane on said support structure and wherein said support structure ispivotally mounted upon a support within a fluid stream.
 13. Apparatusaccording to claim 8, wherein adjacent airfoils are each free tooscillate in both pitch and plunge.
 14. Apparatus according to claim 8,wherein each airfoil includes a pivotally mounted flap adjacent to itstrailing edge.
 15. Apparatus according to claim 14, wherein adjacentairfoils are each free to oscillate in plunge.
 16. Apparatus accordingto claim 14, wherein adjacent airfoils are free to oscillate in pitch.17. Apparatus according to claim 8, wherein said utilizing meansproduces alternating current.
 18. Apparatus according to claim 8,wherein said utilizing means operates a pump.
 19. Apparatus according toclaim 18, wherein said pump includes a second cascade of airfoilspositioned in a moving fluid stream and wherein said utilizing meansoscillates said second cascade in plunge with adjacent airfoilsoscillating substantially 180° out of phase.
 20. Apparatus according toclaim 19, wherein said utilizing means also oscillates said secondcascade in pitch.
 21. Apparatus according to claim 8, wherein saidairfoils are interconnected by said mounting means to form twosubsystems with the airfoils of each subsystem being movable in phaseand with adjacent airfoils in different subsystems.
 22. Apparatusaccording to claim 21, wherein said subsystems are relatively movable inboth pitch and plunge.
 23. Apparatus according to claim 21, wherein saidsubsystems are relatively movable in pitch and wherein each airfoil isprovided with a flap pivotally mounted adjacent to its trailing edge.24. Apparatus according to claim 21, further including a mechanicaloscillating means for each subsystem to disturb said airfoils and tomaintain and enhance the oscillations of said airfoils when theapparatus is subjected to fluid flow at a critical velocity capable ofinducing flutter oscillations.
 25. Apparatus according to claim 21,wherein the subsystems are mechanically interconnected to move 180° outof phase with each other.
 26. Apparatus according to claim 25, furtherincluding mechanical oscillating means to disturb said airfoils and tomaintain and enhance the oscillations of said airfoils when theapparatus is subjected to fluid flow at a critical velocity capable ofinducing flutter oscillations.
 27. Apparatus according to claim 8,further including means for adjusting the conditions under which saidairfoils will begin to oscillate.
 28. Apparatus according to claim 27,wherein said adjusting means includes at least one movable mass mountedon said mounting means.
 29. Apparatus according to claim 27, furtherincluding a control means for maintaining the oscillatory movement whenthe velocity of the fluid stream varies, said control means including adetector and feedback means for controlling said adjusting means inresponse to a detection signal.
 30. Apparatus according to claim 29,wherein said detector includes means for detecting the amplitude of theairfoil oscillations.
 31. Apparatus according to claim 29, wherein saiddetector includes an anemometer.
 32. Apparatus for converting thekinetic energy of a fluid stream into useful work comprising a supportstructure open at opposite ends to permit fluid flow therethrough, aplurality of thin airfoils of high aspect ratio and which arerectangular in plan form, means for mounting said airfoils within saidsupport structure so that they are equally spaced in a cascadearrangement at zero angle of attack when undisturbed and so that eachairfoil has at least two degrees of freedom and adjacent airfoils aremovable out of phase, said airfoils being arranged in two subsystemswith the airfoils of each subsystem being movable in phase, oscillatingmeans for disturbing at least one of said subsystems and for maintainingand enhancing the oscillations of said airfoils when the fluid flowingthrough said device is at a critical velocity sufficient to induceharmonic oscillations due to flutter, control means responsive tovariations in flow velocity to vary the parameters of said apparatus sothat the prevailing velocity is a critical velocity sufficient to induceflutter oscillations, and means for utilizing the flutter oscillationsto produce useful work.
 33. A method for increasing the propulsion of amoving fluid stream comprising the steps of positioning a cascade ofairfoils in a bounded fluid stream and applying a mechanical forcethereto to oscillate said airfoils in a direction having a componentsubstantially perpendicular to the direction of fluid flow with adjacentairfoils oscillating substantially 180° out of phase.
 34. A methodaccording to claim 33, wherein said airfoil is oscillated in plunge. 35.A method according to claim 34, wherein said airfoil is also oscillatedin pitch.
 36. A method for increasing the propulsion of a moving fluidstream comprising the steps of positioning a cascade of airfoils withina moving fluid stream and applying a mechanical force to oscillate saidairfoils in a direction having a component substantially perpendicularto the direction of fluid flow with adjacent airfoils oscillatingsubstantially 180° out of phase.
 37. A method according to claim 36,wherein said airfoils are oscillated in plunge.
 38. A method accordingto claim 37, wherein said airfoils are also oscillated in pitch. 39.Apparatus for increasing the propulsion of a moving fluid streamcomprising a cascade of airfoils and mechanical driving means foroscillating said plurality of airfoils so that adjacent airfoils areoscillated substantially 180° out of phase.